An aircraft may include a lubrication system to provide lubricants to certain components. Typically, the lubrication system includes at least an oil reservoir, an oil supply pump, and an oil scavenge pump. The oil supply pump may be configured to pump oil from the oil reservoir to a target component, and the oil scavenge pump may be configured to pump oil out of the target component back to the oil reservoir.
In many instances, the lubrication system is integral to, and supplies oil for, an aircraft engine, such as an auxiliary power unit, to lubricate gears, a drive shaft, and an engine shaft that are disposed at least partially in the engine or in an integral accessory gearbox. In these configurations, the oil supply pump and oil scavenge pump may be mounted to the drive shaft to be driven by the engine. When the drive shaft rotates, the oil supply pump draws oil from the oil reservoir and pumps the oil to the gearbox and engine, and the oil scavenge pump removes oil from the gearbox and engine and returns it to the oil reservoir. Typically, an oil cooler is disposed between the oil scavenge pump and the oil reservoir to thereby cool the oil before it is returned to the oil reservoir.
In some cases, oil is maintained in certain aircraft components, such as in the oil cooler, in order to comply with federal aviation regulations. In this regard, flow through the lubrication system is controlled by a check valve disposed between the oil scavenge pump and the oil cooler. The check valve is typically configured to open when the pressure of the oil being pumped through the oil cooler is greater than a predetermined pressure magnitude. In many instances, the predetermined pressure magnitude is relatively high (e.g., about 10 psi), so that oil can remain within the oil cooler during engine shutdown.
Although oil is suitably maintained in the oil cooler during engine operation, the above-mentioned lubrication system configuration has drawbacks. For example, when the engine is in a “forward windmill condition” (e.g., where the engine is shut down and unwanted inlet air flow impinges on an engine fan/compressor to turn the engine shaft), the engine shaft may rotate at low speeds, such as about 1% to about 3% of full engine speed. The engine shaft drives the oil supply/oil scavenge pump drive shaft. However, because the shaft rotational speeds are low during the windmill condition, the pressure generated by the oil scavenge pump may be inadequate (e.g., less than 10 psi) to cause the check valve to open. Thus, the rotating drive shaft may cause the oil supply pump to pump oil into the engine and gearbox, while the oil scavenge pump may be prevented from drawing oil out of the gearbox. Consequently, a volume of oil may remain in the gearbox, which may cause the oil reservoir to appear to be low. If oil is added to the oil reservoir, overboard oil leakage through a gearbox vent, inlet oil ingestion, and/or a cabin odor issue may result.
Therefore, there is a need for systems and methods that prevent oil migration into the gearbox when the engine is shut off and the engine shaft is rotating. Moreover, it is desirable for the systems to be capable of being retrofitted into existing engines. Additionally, it is desirable for the systems to be relatively inexpensive and simple to install.